[llvm] [llvm][utils] Add a script to use PRs over pushing to main (PR #166473)

[Paul Kirth via llvm-commits]

ilovepi wrote:

I added a documentation file, since I figured it would be useful. Not sure I put it in a great place, but `llvm/docs` didn't quite feel right. Happy to move to a better place.

https://github.com/llvm/llvm-project/pull/166473